Introduction: Particles at fluid–fluid interfaces

Particle adsorption at the interface between two immiscible fluids is a common phenomenon that plays an important role in various technological processes.^{Reference Binks and Horozov1} For example, the mining industry has taken advantage of particle attachment to air bubbles to separate and recover valuable minerals, a process known as froth flotation.^{Reference Trahar and Warren2,Reference Crawford and Ralston3} Interfacial attachment of colloidal particles to oil–water or air–water interfaces has been used to stabilize emulsions and foams in the food, pharmaceutical, and cosmetics industries.^{Reference Binks and Horozov1,Reference Dickinson4–Reference Frelichowska, Bolzinger, Valour, Mouaziz, Pelletier and Chevalier6} More recently, particle-stabilized emulsions are being explored for applications in enhanced oil recovery and biofuel conversion processes.^{Reference Sullivan and Kilpatrick7,Reference Crossley, Faria, Shen and Resasco8}

For materials scientists and engineers, fluid–fluid interfaces provide unique opportunities to direct the assembly of micro- and nanoscale building blocks to generate structures with desired properties and functionality.^{Reference Binks and Horozov1} The entrapment of particles at a planar air–water interface and their subsequent transfer onto a solid surface (also known as the Langmuir–Blodgett method) has long been used as a versatile method to generate highly uniform layers of nanoscale materials.^{Reference Roberts9} Moreover, the interfaces of emulsion droplets and bubbles provide versatile templates to direct the assembly of particles of various shapes, compositions, and wettability characteristics into three-dimensional structures.^{Reference Brugarolas, Tu and Lee10–Reference Shao, Lv and Yang15} Exquisite control over the shape and chemistry of particles, enabled by recent advances in materials chemistry, provides unprecedented possibilities for enabling the hierarchical assembly of nanomaterials through the interfacial assembly of particles.^{Reference Studart, Gonzenbach, Akartuna, Tervoort and Gauckler16} In particular, particles with complex geometries and surface chemistries that emulate the complexity of molecules such as proteins and amphiphilic surfactants offer endless possibilities for directing the assembly of materials at fluid interfaces.^{Reference Liu, Wei, Qu and Yang17–Reference Ruhland, Gröschel, Walther and Müller21}

Here, we briefly introduce the fundamental ideas behind the attachment and configuration of micro- and nanoscale particles at fluid–fluid interfaces and describe how the shapes and wetting properties of particles affect their configurations, interactions, and assembly at fluid interfaces. Significant focus is placed on the behavior of particles that have both chemical and geometric anisotropy at an interface between water and an immiscible fluid phase such as oil or air. It is critical to understand the interfacial behavior of particles at the single-particle level because their geometry and wettability characteristics directly influence configurations and interparticle interactions and, consequently, the microstructures and mechanical properties of particle-laden fluid interfaces. We also provide examples of the exploitation of particle attachment to fluid interfaces to direct the assembly of nanoscale materials to generate functional structures, in addition to providing an outlook on some of the outstanding challenges in this area.

Adsorption and configuration of particles at fluid–fluid interfaces

Colloidal particles that do not have extreme wetting properties attach strongly to interfaces between two immiscible fluid phases, one of which is typically water (e.g., water–oil or water–air).^{Reference Binks and Horozov1,Reference Binks22} This strong tendency of particles to attach and remain at interfaces between water and another fluid phase can be rationalized by estimating the energy that would be required to remove a particle that resides at the interface. For a spherical particle, this detachment energy can be expressed as:^{Reference Binks22}

(1)$$\Delta E = {\rm{\pi }}r^2 {\rm{\gamma }}_{{\rm{AB}}} \left( {1 - \cos {\rm{\theta }}} \right)^2 ,\,$$

where r, γ_AB, and θ denote the particle size, the interfacial tension between the two fluids, and the three-phase contact angle of the particle, respectively (Figure 1). The three-phase contact angle (θ), determined through one of the two fluid phases at the point where the fluid–fluid interface contacts the solid surface, is a critical parameter that determines the behavior of particles at interfaces. For a spherical nanoparticle with a 10 nm diameter and neutral wetting properties (θ = 90°), the detachment energy from a typical oil–water interface is three orders of magnitude greater than thermal energy (k _BT), which indicates that, once the particle is attached to the interface, it prefers to stay there. This tendency can be understood by considering the reduction in the interfacial area between the two immiscible fluid phases due to the attachment of the particle. It is important to note that, as particle size and density increase, gravity starts to play an important role in the interfacial behavior of the particle. In this review, we limit our discussion to those cases in which the effect of gravity is negligible.

Figure 1.

Schematic illustration of chemically homogeneous particles at fluid–fluid interfaces.

Particles adsorbed at a fluid–fluid interface adopt configurations that depend on their chemical and geometric characteristics.^{Reference Kumar, Park, Tu and Lee18} For spherical particles with uniform wettability, the position with respect to the fluid interface is determined solely by the three-phase contact angle (θ), as shown in Figure 1.^{Reference Binks22} In contrast, the configuration of nonspherical particles (e.g., ellipsoids, dumbbells, and cylinders) depends on the particle shape as well as the surface wettability.^{Reference Madivala, Fransaer and Vermant23–Reference Lewandowski, Searson and Stebe25} In general, particles with high anisotropies (i.e., large aspect ratios) tend to lie horizontally at the fluid interface with their major axes parallel to the fluid interface while satisfying the wetting conditions. This preference of anisotropic particles to adopt horizontal orientations is due to the tendency of the system to maximize the interfacial area eliminated by the placement of the particles (S _I) at the fluid interface (i.e., minimize the total interfacial area between the two fluid phases).

More recently, particles that have both chemical and geometric anisotropy/heterogeneity have been synthesized, and their interfacial behavior has received significant attention.^{Reference Kumar, Park, Tu and Lee18,Reference Walther and Muller19,Reference Glotzer and Solomon26} So-called “patchy” particles that have nonuniform wetting properties and nonspherical shapes could potentially serve as colloidal atoms and molecules that enable directional bonding and hierarchical assembly.^{Reference Kumar, Park, Tu and Lee18,Reference Glotzer and Solomon26,Reference Chen, Whitmer, Jiang, Bae, Luijten and Granick27} Proteins, in fact, could be considered to be the most complex examples of such patchy particles. Janus particles are a class of patchy particles that have two regions with opposite polarities or wettabilities. The name Janus derives from the two-faced Roman god of beginning and end.^{Reference de Gennes28} The interfacial behavior of these particles is especially intriguing, because the structure of these particles is analogous to that of amphiphilic molecules, which are known to preferentially segregate to fluid interfaces (Figure 2).

Figure 2.

Schematic illustrations of emulsion droplets stabilized by amphiphilic molecules and Janus particles.

The configuration of a Janus particle at a fluid–fluid interface can be represented by the orientation of the particle with respect to the fluid interface and the distance between the center of mass of the particle and the interface.^{Reference Binks and Fletcher29–Reference Park and Lee35} Two critical factors determine the equilibrium configuration of Janus particles at fluid–fluid interfaces: the tendency of the particles to maximize the surface area of preferred contact between the particle surfaces and the fluid phases (i.e., between the apolar [A] surface of the particle and the oil [O] phase, S _AO, and between the polar [P] surface of the particle and the water [W] phase, S _PW) and the tendency of the particles to maximize the interfacial area displaced as a result of the presence of the particle at the interface (S _I), as illustrated in Figure 3. For Janus spheres, the first factor (i.e., the preferred wetting) plays a dominant role in determining the equilibrium configuration.^{Reference Binks and Fletcher29–Reference Jiang and Granick31,Reference Fan, Resasco and Striolo36} In general, a Janus sphere at equilibrium adopts a configuration in which each hemisphere is fully exposed to its preferred fluid phase and, thus, the boundary between the two hemispheres (Janus boundary) coincides with the interface. As the distance between the Janus boundary and the particle center increases and/or the wettability difference between the two sides decreases, however, the Janus boundary can be displaced from the fluid–fluid interface.^{Reference Ondarçuhu, Fabre, Raphaël and Veyssié37}

Figure 3.

Schematic behavior of Janus particle configurations at a water–oil interface, depending on the aspect ratio (AR). The subscript “A” denotes the apolar surface of the particle, and “P” is the polar surface; “O” is oil and “W” is water. S _AO represents the contact area between the apolar surface of the particle and the oil phase. θ_r,eq indicates the orientation angle at an equilibrium state.

In the case of particles with both geometric and chemical anisotropy (i.e., nonspherical Janus particles), the effect of the second factor (i.e., energy reduction with increasing displaced area [S _I]) on the particle configuration becomes significant and competes with the preferential wetting tendency to determine the configuration of the particle (Figure 3).^{Reference Park and Lee34,Reference Park and Lee35,Reference Rezvantalab and Shojaei-Zadeh38} For example, Janus ellipsoids can adopt either upright or tilted configurations, depending on the aspect ratio (AR) of the particle and the difference in wettability of the two regions. One way to illustrate the effect of the wettability difference is to consider a Janus ellipsoid that has two regions with supplementary contact angles; that is, the polar and apolar sides have contact angles of θ_P = 90° – β and θ_A = 90° + β, respectively (see Figure 4), where 2β represents the difference in the wettability of the two regions. Thus, a large value of β represents a large difference in the wettabilities of the two sides. Based on numerical calculations, it has been shown that Janus ellipsoids with small AR and large β values prefer to adopt the upright orientation (i.e., with the major axis normal to the plane of the fluid interface) to maximize the preferential wetting (i.e., pink region in Figure 4). In contrast, particles with large AR and small β values tend to adopt a tilted orientation, which maximizes the displaced interfacial area, as illustrated in Figure 4.

Figure 4.

Orientation phase diagram for Janus ellipsoids with α = 90° and supplementary wettability β. The Janus ellipsoids adopt the upright or tilted orientations, depending on the values of β and AR_e. Reproduced with permission from Reference 34. © 2012 American Chemical Society.

Interestingly, under certain conditions, highly anisotropic Janus particles can adopt either upright or tilted orientations (i.e., light blue and yellow regions in Figure 4) as a result of the existence of two energy minima in the attachment energy profile as a function of the orientation angle.^{Reference Park and Lee34,Reference Park and Lee35} The energy barrier that exists between these two states typically tends to be much greater than the thermal energy, so a collection of highly anisotropic Janus particles can exhibit coexistence of the two orientations. The transition of a Janus particle from the metastable to the equilibrium configuration can occur spontaneously when the particle size is sufficiently small (<10 nm) so that the magnitude of the energy barrier between the two configurations is comparable to the thermal energy.^{Reference Park and Lee34} The coexistence of tilted and upright orientations in anisotropic Janus particles has been demonstrated in both theoretical and experimental studies.^{Reference Park, Choi, Kang, Tettey, Lee and Lee32–Reference Park and Lee35}

Interactions and assemblies at fluid–fluid interfaces

Chemically isotropic spheres

Two types of interactions, one repulsive and one attractive, dominate the interactions between particles and consequently affect their microstructure at fluid–fluid interfaces. The repulsive interaction is due to electrostatics caused by charge dissociation in polar or apolar solutions.^{Reference Park, Vermant and Furst39–Reference Oettel and Dietrich46} Theoretical studies and experimental observations have shown that the resulting force scales as F _rep ∼ r ^–4, where r is the center-to-center separation between two particles. The attractive interaction is caused by interface deformation around individual particles.^{Reference Oettel and Dietrich46–Reference Stamou, Duschl and Johannsmann50} Ideally, for perfectly spherical particles with uniform surface properties, one would not expect any type of interface deformation around the particles (i.e., the fluid interface would remain planar around spherical particles). For typical colloidal particles, however, chemical and geometric heterogeneities of the particle surface lead to deformation of the fluid interface.^{Reference Lewandowski, Cavallaro, Botto, Bernate, Garbin and Stebe24,Reference Park, Choi, Kang, Tettey, Lee and Lee32,Reference Park, Choi, Kang, Tettey, Lee and Lee33,Reference Botto, Lewandowski, Cavallaro and Stebe51–Reference Loudet and Pouligny55} Particles are able to lower the free energy of the system by coming together and reducing the interfacial area; in other words, particles experience attractive interactions. The resulting capillary force can be approximated by quadrupolar interactions and decays as F _att ∼ r ^–5.^{Reference Danov, Kralchevsky, Naydenov and Brenn47,Reference Stamou, Duschl and Johannsmann50}

The relative strengths of the electrostatic repulsions and capillary attractions determine the assembly behavior and, consequently, the microstructure of a collection of particles at fluid–fluid interfaces.^{Reference Reynaert, Moldenaers and Vermant56} For example, spherical polystyrene particles with surface sulfate groups at a water–oil interface exhibit dominantly strong repulsions and form a hexagonal lattice structure with large interparticle distance that can be several times the particle size (Figure 5a). The addition of electrolytes and/or surfactants to the fluid phase can decrease the Debye screening length and/or alter the wettability of the particles at the interface. Such changes reduce electrostatic repulsion, so that attraction becomes dominant, resulting in the formation of aggregates or percolated networks (Figure 5b).^{Reference Park, Pantina, Furst, Oettel, Reynaert and Vermant44}

Figure 5.

Interactions and microstructures of homogeneous polystyrene spheres with a diameter of 2a ≈ 3 μm at a water–oil interface. (a–b) Microstructures (a) without and (b) with 0.25 M NaCl in the aqueous phase. (c) Measurement of pair interactions using optical laser tweezers. (d) Heterogeneous pair interaction forces (F) as a function of separation (r). The green curves indicate pair interaction force profiles in the absence of any additives in the fluid phase. The interaction force decreases in the presence of 0.25 NaCl in the aqueous phase: the black and red curves show the repulsion- and attraction-dominant force profiles, respectively, whereas the blue curves are intermediate between them. The dashed lines indicate the guide lines for F ∼ r ^–4. (a–b) Reproduced with permission from Reference 56. © 2006 American Chemical Society. (c–d) Reproduced with permission from Reference 49. © 2011 Royal Society of Chemistry.

Interaction forces between two spherical particles at a water–decane interface were directly measured using optical laser tweezers (Figure 5c).^{Reference Park, Vermant and Furst39,Reference Park, Pantina, Furst, Oettel, Reynaert and Vermant44,Reference Park and Furst49,Reference Park and Furst57,Reference Park and Furst58} Interestingly, the interactions were found to be highly heterogeneous (Figure 5d). In the presence of salt and/or surfactants, the pair interaction forces showed a significant reduction in magnitude, but nevertheless exhibited substantial heterogeneity. The origin of such heterogeneous interactions is thought to be surface charge nonuniformity.^{Reference Park, Vermant and Furst39,Reference Feick, Chukwumah, Noel and Velegol59} Heterogeneity in interactions between particles at a fluid interface plays an important role in determining the microstructure and micromechanical properties of the particle-laden interface.^{Reference Park, Vermant and Furst39,Reference Park and Furst60}

Chemically isotropic and geometrically anisotropic particles

Satisfying the wetting condition on the surface of chemically homogenous but geometrically anisotropic particles can lead to the deformation of the fluid interface surrounding the particles. This interface deformation makes the capillarity-induced attractive interactions significantly larger than the electrostatic repulsions.^{Reference Botto, Lewandowski, Cavallaro and Stebe51} For example, ellipsoidal particles showed quadrupolar interface deformation of the air–water interface; specifically, interface depression and ascension were found around the tip and side regions, respectively, of an ellipsoid, as shown in Figure 6a. Capillarity-induced attractions between these ellipsoids subsequently affect their assembly structures, which consist of either side-to-side or tip-to-tip assemblies.^{Reference Loudet, Alsayed, Zhang and Yodh54} By taking advantage of the tendency of ellipsoids to form a strong network through capillary attraction, Yunker et al. showed that nonuniform deposition of particles from a drying suspension, also known as the “coffee ring” effect, can be suppressed.^{Reference Yunker, Still, Lohr and Yodh61}

Figure 6.

Simulated interface deformation of chemically isotropic (a) ellipsoids and (b) cylinders, and optical images of their resulting microstructures at the water–oil and water–air interfaces, respectively. The diagrams in panels a–b indicate the magnified interface profiles around the ellipsoid and cylinder, respectively. Optical image in (a) reproduced with permission from Reference 23. © 2009 American Chemical Society. Remaining images reproduced with permission from Reference 52. © 2012 Royal Society of Chemistry.

Chemically homogeneous cylindrical particles at oil–water and air–water interfaces also exhibit quadrupolar interface deformation similar to ellipsoids.^{Reference Lewandowski, Cavallaro, Botto, Bernate, Garbin and Stebe24,Reference Botto, Lewandowski, Cavallaro and Stebe51,Reference Botto, Yao, Leheny and Stebe52} It was observed that the interface deflects upward on the planar end surfaces and downward around the sides for cylinders with three-phase contact angles of less than 90° (Figure 6b). Because of the prominent interface deformations on the two end surfaces, cylinders predominantly assemble in tip-to-tip orientation, resulting in the formation of linear chain-like structures. Interestingly, these cylindrical particles can be steered to specific locations by controlling the interface curvature and gradients using microfabricated posts on solid surfaces. In this case, the interface gradients act as an external field to direct the assembly of particles at the fluid interface, potentially enabling a novel approach for engineering material structures and properties.^{Reference Cavallaro, Botto, Lewandowski, Wang and Stebe62}

Janus particles: Particles with geometric and chemical anisotropy

The configuration of Janus particles at fluid–fluid interfaces, which depends on their shape and surface wettability, has a significant impact on the lateral interactions among these particles at fluid interfaces (Figure 7). Janus spheres composed of strong apolar (DDT) and sulfate-coated polar (PS) hemispheres, for example, exhibit quadrupolar capillary attractions between two particles.^{Reference Park, Brugarolas and Lee63} The corresponding microstructure has consistently been found to be capillarity-induced aggregated networks (left panel in Figure 7a). It is believed that the capillary attraction is due to the undulation of the contact line around the diffuse Janus boundary between the two hemispheres. In contrast, when Janus spheres contain two negatively charged-polar hemispheres, the electrostatic repulsion becomes dominant, and a two-dimensional hexagonal array of particles resembling an array of homogenous spheres is achieved (right panel in Figure 7a). The similarity of the surface properties of the two hemispheres likely reduces the contact-line undulation, making electrostatic repulsion more dominant.

Figure 7.

(a) Configurations and microstructures of spherical Janus particles at the oil–water interface depending on surface modification.^{Reference Park, Brugarolas and Lee63} DDT-Au-PS and MPA-Au-PS represent gold-coated polystyrene (Au-PS) Janus particles in which the gold hemisphere surface is modified with 1-dodecanethiol (DDT) and 3-mercaptopropionic acid (MPA), respectively. (b) Optical profilometry image showing asymmetric quasi-quadrupolar interface deformation around a Janus cylinder in a tilted orientation at the air–water interface and assembly structure of Janus cylinders.^{Reference Park, Choi, Kang, Tettey, Lee and Lee33} (c) Assemblies of amphiphilic rods at the oil–water interface.^{Reference Wang, Wang, Sheiko, Betts and DeSimone64} The scale bars are 200 μm. (a) Reproduced with permission from Reference 63. © 2011 Royal Society of Chemistry. (b) Reproduced with permission from Reference 33. © 2013 American Chemical Society. (c) Reproduced with permission from Reference 64. © 2011 American Chemical Society.

In addition to chemical anisotropy, particle shape is also a critical factor that determines the interactions and subsequent assembly behavior of Janus particles at fluid interfaces. These particle behaviors are significantly different from those of chemically homogenous ellipsoids and cylinders that exhibit highly deterministic interactions and assembly behaviors. For instance, when a Janus cylinder adopts a tilted orientation at an air–water interface, the shape of the interface deformation shows an asymmetric quasi-quadrupole, which, in turn, significantly affects the pair interactions (Figure 7b).^{Reference Park, Choi, Kang, Tettey, Lee and Lee33} Depending on the lateral alignments of two particles with respect to each other as they approached, the scaling exponent (λ) for the capillary force (F _att ≈ r ^λ) was found to be between –6.8 and –5.4, indicating very complex attractive interactions.

Complex multipolar interface deformations around anisotropic Janus particles lead to nondeterministic assembly structures, and a wide variety of structures have been observed for these particles at fluid interfaces. Interestingly, when anisotropic particles with centrosymmetric wettability patterns are spread at fluid–fluid interfaces, the particles lie horizontally at the fluid interface similar to chemically homogeneous anisotropic particles, as shown in Figure 7c. The patchiness of the surface wettability of these particles, however, leads to a diverse assembly behavior that depends on the relative position and size of each patch.^{Reference Wang, Wang, Sheiko, Betts and DeSimone64}

Directed assembly of particles at fluid–fluid interfaces: Approach to generate hierarchical structures

As recognized by Langmuir and Blodgett in the early 1900s, the entrapment of particles at a planar air–water or oil–water interface allows one to transfer the interfacial film onto a solid surface to create a functional thin film.^{Reference Roberts9} This method has long been used by the materials research community to generate thin films with unique optical, electrical, and photonic properties.^{Reference Kuhn, Möbius, Bücher, Weissberger and Rossiter65} More recently, multiphasic fluid mixtures such as emulsions and bubbles have been used as templates to direct the assembly of various materials and generate three-dimensional structures.^{Reference Brugarolas, Tu and Lee10} Through careful control of the wettability of particles, emulsion droplets and gas bubbles can be stabilized to generate functional hollow shells (Figure 8a).^{Reference Binks and Murakami66–Reference Lee and Weitz68} These hollow shells represent three-dimensional structures that can be further assembled (Figure 8b). In addition, emulsion droplets and gas bubbles with complex structures, such as a “drop-in-drop” geometry (Figure 8c), and those generated using microfluidic methods provide unique opportunities to generate hierarchical structures with control over multiple length scales.^{Reference Brugarolas, Tu and Lee10,Reference Murakami, Moriyama, Yamamoto, Binks and Rocher13}

Figure 8.

(a) Assembly of particles at water–fluid interfaces, depending on the three-phase contact angle of particles.^{Reference Binks and Murakami66} (b) Schematic illustration of hierarchical structures generated from the particle assemblies shown in (a).^{Reference Brugarolas, Tu and Lee10} (c–d) Complex structures formed using multiple emulsions/bubbles as templates.^{Reference Murakami, Moriyama, Yamamoto, Binks and Rocher13} (c) Schematic illustration and (d) snapshot showing the preparation of oil-in-water-in-air powered emulsions. (a) Reproduced with permission from Reference 66. © 2006 Nature Publishing Group. (b) Reproduced with permission from Reference 10. © 2013 Royal Society of Chemistry. (c–d) Reproduced with permission from Reference 13. © 2012 Wiley-VCH.

An advantage of droplet-/bubble-directed assembly is that the emulsions and foams can have multifunctional properties depending on the functionality of particles that are used for stabilization. One exciting development in this direction is the use of stimuli-responsive particles. Responsiveness to stimuli is particularly important in the encapsulation and delivery of molecules, as well as the formation of three-dimensional structures that can completely change their shapes and properties. For example, Lam and co-workers reported stimuli-responsive Pickering foams (i.e., foams stabilized by solid particles) that are stable under ambient conditions but can be destroyed in the presence of a magnetic field (Figure 9).^{Reference Lam, Blanco, Smoukov, Velikov and Velev11,Reference Blanco, Lam, Smoukov, Velikov, Khan and Velev69} Such magneto-Pickering foams, stabilized by a mixture of magnetic and nonmagnetic particles, are potentially useful in a wide range of applications related to noncontact defoaming processes in fermentation and wood pulp production. Similarly, multi-stimuli-responsive materials have been prepared in which magnetic particles and thermoresponsive fatty acid tubes were used to generate foams. The obtained foams essentially showed a tuneable stability that was responsive to thermal, light, and magnetic stimuli.^{Reference Fameau, Lam and Velev70}

Figure 9.

(a) Fabrication of magnetoresponsive foams stabilized by magnetic particles.^{Reference Lam, Blanco, Smoukov, Velikov and Velev11} (b) Tunable stability of such foams under a magnetic field.^{Reference Blanco, Lam, Smoukov, Velikov, Khan and Velev69} In the schematic illustration, R is the radius of an air bubble, ΔP is the Laplace pressure inside the bubble, γ is the surface tension between air and water, F _drag is the drag force of the bubble, and F _mag is the magnetic force exerted on a magnetic particle. (a) Reproduced with permission from Reference 11. © 2011 American Chemical Society. (b) Reproduced with permission from Reference 69. © 2013 American Chemical Society.

Interfacial attachment of particles also provides a versatile approach to direct the assembly of nanostructured carbons, such as graphene and carbon nanotubes, to generate three-dimensional macroscopic structures with various functionalities. As for colloidal particles, it was reported that graphene oxides (GOs) are surface-active and form stable Pickering emulsions (Figure 10a).^{Reference Kim, Cote, Kim, Yuan, Shull and Huang12} Hollow graphene oxide particles were prepared by stabilizing water droplets in oil and subsequently removing the water (Figure 10b).^{Reference Guo, Song and Chen14} These graphene hollow capsules showed excellent electrochemical properties when used as anode materials in lithium-ion batteries. Three-dimensional ordered networks from reduced graphene oxides (RGOs) have also been fabricated by self-assembly at oil–water interfaces (Figure 10c and d).^{Reference Lee, Kim, Hwang, Lee, Kwon, Bielawski, Ruoff and Kim71} RGO platelets dispersed in a volatile organic solvent were exposed to a stream of humid air, and spontaneous condensation of aqueous droplets occurred at the organic solution surface. RGOs assembled at the water–solvent interface eventually form macroporous RGO-based films upon complete drying of the solvent. The obtained films were found to be mechanically flexible but robust, while also exhibiting improved electrical properties and high chemical reactivity.

Figure 10.

Assembly of nanostructured carbons (e.g., graphene and carbon nanotubes) at water–fluid interfaces. (a) Photographs and optical microscopy images of stable Pickering emulsions (i.e., toluene droplets in water) covered by graphene oxide; the concentration of graphene oxide was varied: (1) 0.95, (2) 0.47, (3) 0.19, (4) 0.095, (5) 0.047, (6) 0.019, and (7) 0.0095 mg/mL.^{Reference Kim, Cote, Kim, Yuan, Shull and Huang12} (b) SEM image of hollow graphene oxide capsules.^{Reference Guo, Song and Chen14} (c) Carbon film formation with three-dimensional ordered networks using reduced graphene oxide.^{Reference Lee, Kim, Hwang, Lee, Kwon, Bielawski, Ruoff and Kim71} (d) The resulting scanning electron microscope image of the carbon film with closely packed macropores. (a) Reproduced with permission from Reference 12. © 2010 American Chemical Society. (b) Reproduced with permission from Reference 14. © 2010 Royal Society of Chemistry. (c–d) Reproduced with permission from Reference 71. © 2010 Wiley-VCH.

Summary and outlook

It is critical to understand the role of particle shape and surface chemistry in the configuration of individual particles, which, in turn, has a significant influence on their assembly behavior and the macroscopic properties of particle-laden interfaces. Considering recent advances in the synthesis of particles with complex shapes and patchiness, it is fair to say that researchers have only begun to scratch the surface of all the possibilities. To fully realize the promise that interfacial assembly of particles offers, several fundamental breakthroughs must be made.

Currently, there are no a priori ways to design and engineer particles that will assemble into specific structures at fluid–fluid interfaces. Most studies to date have relied on synthesizing new types of particles and then subsequently studying their interfacial behaviors. An ability to design particles that will assemble into (predetermined) target structures would be highly desirable. One possible way to enable such a design principle is to apply the so-called inverse materials design methodology using computational and theoretical methods.^{Reference Torquato72}

Another important development that will have broader impact beyond the materials community is the use of patchy particles as model systems to understand the assembly of complex biological molecules such as proteins at fluid interfaces. By designing patchy particles with complex structures and surface properties, it will be possible to systematically test and understand how complex molecules such as proteins assemble into intricate structures such as those found in nature. It will be equally important to keep in mind that differences might arise in the behavior of molecules and particles when they exist at fluid interfaces. Uncovering such differences will be as important as identifying similarities.